We are now able to develop mouse embryos in vitro continously from two-cell eggs to the early somite stage which is equivalent to an embryo of 9 days gestation (Hsu, 1972; Hsu, 1973). The average gestation period of the mouse is 19 days. In order for such a long differentiation to occur continuously in vitro; two factors in embryonic sera are required. On the basis of these results, the aberrant differentiation of embryonal carcinoma will be studied. The embryoid body derived from testicular teratocarcinoma of mice resembles mouse embryos of early 5 days gestation ontogenically and morhogenically. The embryoid body differentiates in vitro in the same media in which normal mouse embryos develop. The embryonal carcinoma differentiates to well-organized tissues in vitro within 4 to 6 days of incubation and continuously differentiates for more than a month in vitro. In contrast, th embryoid body continuously duplicates as an undifferentiated and disoganized cell mass without further differentiation. Therefore, our objective would be to study the factor in embryonic sera which promotes the differentiation of embryonal carcinoma, and the factor in the peritoneal cavity which inhibits the differentiation of embryonal carcinoma. The aberrant differentiation of embryonal carcinoma will be studied in vitro in comparison with that of normal mouse embryos by time-lapse cinephotomicroscopy. If possible, this project may lead to an attempt to regulate the "differentiation inhibitor" in vivo or to use the differentiation mediator to control embryonal carcinoma in mice.